John Crane is a globally recognized, and trusted brand and has become a world-renowned global leader. John Crane offers the most complete engineering mechanical seal selection and sealing support systems for pumps, compressors and other rotating devices. At the same time, it is complemented by a large range of engineering bearings and energy transfer couplings. All of the above are provided by John Crane and its worldwide service network.

Mechanical seals are one of the main consumables of centrifugal pumps, and the failure of the pump is mainly caused by seal failure. According to statistics, the failure of the mechanical seal caused the failure of the pump to account for more than 50% of the equipment failure rate. Therefore, it is necessary to analyze and improve the leakage of the mechanical seal of the centrifugal pump to reduce the failure rate of the pump.

1.The working principle of centrifugal pump and the role of mechanical seal

The centrifugal pump passes through the motor and the coupling, and its impeller rotates at a high speed. The liquid between the blades is pushed by the blades. Under the action of centrifugal force, it is continuously broken from the center to the surroundings. When the liquid in the pump is thrown from the center of the impeller, a low pressure zone is formed at the center when the edge turns to the edge. Since the liquid pressure in front of the pump is higher than the pressure at the pump suction port, the liquid is continuously sucked into the pump through the suction pipe under a pressure difference to replenish the discharged liquid. Position, as long as the impeller rotates continuously, the liquid is continuously sucked in and discharged.

When the pump shaft passes through the pump casing, when there is a gap between motion and rest, the liquid in the pump will leak from the gap to the outside of the pump. If the suction end is vacuum, external air will leak into the pump, which seriously affects pump operation. To reduce leakage, shaft end seals are typically installed at static and dynamic gaps. The shaft sealing device is an important part of the centrifugal pump and is directly related to whether the centrifugal pump can work normally. Shaft seals are now usually mechanically sealed.

2.The principle of mechanical seal

The mechanical seal is an axial face seal that relies on the resilient element to pre-tension the stationary ring face seal pair and compress the medium pressure and spring element pressure to achieve a seal.

Mechanical seals typically consist of a stationary ring, a moving ring, a pressing element and a sealing element. The end faces of the moving ring and the retaining ring constitute a pair of friction pairs, and the moving ring is pressed against the end face of the fixing ring by the pressure of the liquid in the sealing chamber, and an appropriate specific pressure is applied to the end faces of the two rings, the layer remaining extremely thin. The liquid film achieves the purpose of sealing.

The pressing element creates a pressure that maintains the end of the pump in an inoperative state, ensuring that the sealing medium does not leak and prevents impurities from entering the sealing end face. The seal ring acts as a gap between the seal ring and the shaft. The gap between the static ring and the gland acts as a buffer for the pump and vibration. Mechanical seals are not isolated components in actual operation and are combined with other components of the pump.

3.Mechanical seal auxiliary equipment - Flushing device

Flushing device is the most important auxiliary device for mechanical seal. During operation of the pump, impurities or deposits are formed in the sealed chamber and must be flushed. Otherwise, the precipitation of the particles will precipitate due to crystallization, causing the mechanical seal to move, the static ring to lose its float, and the spring to fail. More serious is that the particles wear and impurities entering the friction pair will increase the wear, resulting in rapid failure of the mechanical seal. Therefore, the pump mechanical seal should normally be flushed.